The antibody has been widely used in medical studies concerning diagnosis and treatment of diseases as well as in biological analyses, because of its property of specifically binding to an antigen (Fung et al., Curr. Opin. Biotechnol., 2001, 12, 65-69, and Heng Zhu et al., Curr. Opin. Chem. Biol., 2001, 5, 40-45). Recently, as an immunoassay, immunosensors have been developed, which require the immobilization of an antibody on a solid support and measure changes in current, resistance and mass, or optical properties or the like (Kim Rogers et al., Affinity Biosensors: Techniques and Protocols, 1998, Vol. 7). Among them, a surface plasmon resonance-based immunosensor making use of optical properties has been commercialized. The surface plasmon resonance-based biosensor provides qualitative information (whether two molecules specifically bind to each other) and quantitative information (reaction kinetics and equilibrium constants), and also performs sensing in real time without the use of labeling, thus being particularly useful for measuring antigen-antibody binding (Myszka, D. G., J. Mol. Recognit., 1999, Vol. 12, 390-408).
*In the immunosensor, it is very important that antibodies are selectively and stably immobilized on a solid support. The techniques for immobilizing antibodies are classified into two categories, physical immobilization and chemical immobilization. The physical immobilization techniques (Ferretti Paynter S S et al., Trends Anal. Chem., 2000, Vol. 19, 530-540) have been minimally used because they cause denaturation of the protein, and the results are less reproducible. In contrast, the chemical immobilization techniques (Nikin Patel et al., Langmuir, 1997, 13, 6485-6490) have been widely used because they show good reproducibility and a wide range of applications, due to their feature of allowing secure binding of proteins through covalent bonding. However, when immobilization of antibodies is performed using a chemical immobilization technique, the antibodies, being asymmetric macromolecules, often lose their orientation and activity to bind to antigens (Bin Lu et al., Analyst, 1996, 121, 29R-32R).
In an attempt to enhance the ability of antibodies to bind to antigens, a support may be used before the antibodies are linked to a solid substrate, and a technology of using protein G as the support is known. However, there is a problem that this protein G itself also loses orientation and its ability to bind to an antibody when bound to the support.
Accordingly, in order to solve such problem, a variety of methods have been suggested. For example, Streptococcal protein G is treated with 2-iminothiolane to perform phosphorylation of the amino acid group of a protein, and then the phosphorylated Streptococcal protein G is immobilized on an antibody. However, this method is directed to phosphorylating the amino groups of amino acids having an amino group (Arg, Asn, Gln, Lys), instead of phosphorylating any specific site, and thus the method results in low specificity and requires additional purification processes after chemical treatments (Y. M. Bae et al., Biosensors and Bioclectronics, 2005, 21, 103-110).